Avalanches of light in nano format

Whether we are jumping on a trampoline, riding a bike or hammering a nail into the wall - invisible forces accompany us at every turn in our everyday lives. But forces do not only surround us in the world we perceive. Even at the microscopic level of our cells, a multitude of tiny mechanical forces are at work, controlling our biological machinery - from cell division to muscle contraction. These forces are the key to understanding these biological processes and could pave the way for the development of new drugs.

Tiny nanoparticles developed by researchers at Columbia University in New York and Lawrence Berkeley National Lab could help with this endeavor in the future. With a size of just a few tens of nanometers - about a thousand times smaller than the diameter of a hair - they have two properties that are desirable for measuring such forces.

Firstly, the particles can be used to measure a wide range of forces - from extremely weak to relatively large, over ten thousand times as strong. It's as if you could use a single ruler to precisely measure both the diameter of a coin and the length of a soccer stadium. This is interesting for processes in which many different forces are used - in the development of an embryo, for example.

Yet the wide range is not that spectacular on its own. There are already other measurement methods that are also capable of doing this. However, they have a decisive disadvantage: to measure forces, they usually require some kind of bulky appendage such as wires or similar. This makes it difficult to insert these sensors into living organisms or to carry out measurements there.

“That’s what’s beautiful here,” explains Natalie Fardian-Melamed, first author of the recently published paper in the journal ‘nature’, “that we’re using light.” This is because the nanoparticles can be both controlled and read out with light - meaning they can be operated remotely.

The “photon avalanche” can be imagined as “similar to a snow avalanche”. “We have a tiny disturbance that leads to something big, something drastic,” explains researcher Fardian-Melamed. Photo: Krzysztof Kowalik on Unsplash

The new sensors are based on a process known as “photon avalanche”. “Just like its name suggests, it's an avalanche,” says Fardian-Melamed, “similar to a snow avalanche. We have a tiny disturbance which is translated to something big, something drastic.”

In this phenomenon, a single particle of light - a photon - sets off a chain reaction in which a large number of other photons are released at the end. This phenomenon was discovered in large crystals more than forty years ago. Since 2021, it has been known that this can also occur in much smaller nanocrystals. The prerequisite for the avalanche process is that charged foreign atoms are incorporated into the crystals. The latest finding: mechanical forces can influence this process - and can therefore be used to measure force.

This discovery was a pure coincidence. “It started as an accident,” recalls Fardian-Melamed.

One day, the physicist makes a control measurement with the nanoparticles in the laboratory. She wants to test whether force could influence the optical signal of the nanoparticles. Using an atomic force microscope, the researcher presses on one of the tiny particles. To her surprise, the optical signal of the particle changes much more than expected.

“I mean, now that's exciting,” explains the researcher today. “But at that moment, I couldn't believe it. I thought 'No, no, this can't be true!”

At the time, the unexpected observation threatens to thwart her originally planned experiments with the nanoparticles. Initially, the scientist therefore hopes that it is a temporary disruptive factor – possibly an effect caused by the material of the atomic force microscope’s tip. In order to gain clarity, the team carries out a series of further measurements.

But no matter what the researchers change back then: the effect remains. A suspicion is increasingly crystallizing: could the force be influencing the avalanche process? The scientists see an opportunity in what seemed to be a problem at first. If the hypothesis is correct, the nanoparticles could perhaps be converted into force sensors.

The breakthrough comes a little later. It's vacation time. Natalie Fardian-Melamed is sitting in the lab. Alone, in the dark. “No one was in the lab, no one was in the building,” she recalls,” and then I pressed on one of these particles. And oh my, in all the darkness there was suddenly light and it got brighter and brighter and brighter.”

It confirms her assumption: the avalanche process in the nanoparticles does depend on the force applied. And it is the starting signal for the further development of the particles into force sensors - which act differently depending on the configuration: Some become brighter when you press on them. Others become darker. Still others change color.

In the long term, the researchers hope that their nanoparticles will be used in living organisms, for example. They could also be useful for technical applications, such as optimizing batteries.

The team is currently working on further improving the nanoparticles by adding self-calibrating functionalities, for example, and exploring their response to other environmental stimuli such as temperature.

Original publication:

Fardian-Melamed, N., Skripka, A., Ursprung, B. et al.

Infrared nanosensors of piconewton to micronewton forces08 January 2025

Nature 637, 70–75 (2025)

doi.org/10.1038/s41586-024-08221-2